Estimation of Probable Maximum Precipitation (PMP) (Case Study on Upper Awash Sub River Basin)
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Date
2011
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Addis Ababa University
Abstract
Probable Maximum Precipitation (PMP) is defined as theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a particular time of year. The probable maximum precipitation helps for the design a civil structure appropriately in the study area. PMP value should be estimated by more than one method for critical hydrologic regulations, and preferably by both statistical and moisture maximization methods. The PMP for rainfall stations in Ethiopia have been estimated by Hershfield statistical method with frequency factor (K) determined by the Hershfield’s chart. But different studies show that the value of frequency factor founded from the chart was not reliable. Therefore the main purpose of this study was to estimate the value of PMP using the frequency factor obtained from both the statistical equation and Hershfield’s chart for 1-day, 2-days and 3-days duration and compare the results. In addition to this estimate the PMP values using both the statistical method and moisture maximization methods for first class meteorological stations in the Upper Awash sub river basin and compare the results. In this paper, yearly maximum one day, two days and three days duration rainfall data of 30 years for 11 stations in the Upper Awash sub river basin were analysed to estimate PMP based on statistical method. The study shows that the maximum frequency factor (Km) founded from the statistical equation is 6.2 and the PMP value obtained using the Hershfield chart maximum frequency factor (Km) deviated around 54% from the PMP Value obtained from the Hershfield’s frequency equation. This result clearly indicated that Hershfield’s chart overestimated PMP value which leads to uneconomical designs in the Upper Awash Sub River Basin. The maximum dew point temperature is very essential parameter to estimate PMP using moisture maximization method and for any location it is chosen by surveying long record period (50 years or above) of a highest value of persisting dew point of the station. But in this study the maximum record period is 30 years, so the maximum dew point temperature is calculated by performing the frequency analysis on annual series of monthly maximum persisting temperature with 100 year return period as a recommendation of (WMO, 2009). According to this paper Hershfield’s Statistical Procedure is well-matched method in the Upper Awash Sub Basin.
Key words: probable maximum precipitation, statistical method, Hershfield’s Statistical procedure, moisture maximization method, Upper Awash Sub River Basin
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Keywords
Probable maximum precipitation, Tatistical method, Hershfield’s Statistical procedure, Moisture maximization method, Upper Awash Sub River Basin